Over the past several years there has been a significant decline in the incidence of Sudden Infant Death Syndrome (SIDS). This decline coincides with worldwide recommendations for caretakers to avoid placing sleepling infants in the prone, face down, position. However, from one month to one year of age, SIDS remains the most common cause of infant mortality, accounting for nearly 30% of deaths through this postneonatal period. The long-term objectives of this protocol are to elucidate physiologic mechanisms that underlie SIDS, and to develop age-appropriate, non-invasive tests that will identify infants who at the greatest risk for SIDS. These tests focus on assessments of peripheral and central mechanisms involved in the integrated control of cardiac, blood pressure, and respiratory function. Subjects are the fetuses and infants of mothers from a low-SES population in New York City and from a rural Native American population on the Pine Ridge Reservation in South Dakota which is at unusually high risk for SIDS. The studies begin with measurements made during late gestation. Assessments then continue through infancy to document the physiologic changes that coincide with the period of maximal risk for SIDS. These postnatal studies incorporate measurements made during sleep under basal conditions as well as during head-up and head-down tilting. The principal dependent variables are heart rate and several indices of heart rate variability, respiratory rate and variability, body temperature, and measures of electrocortical activity, and beat-to-beat blood pressure.